Electrical cables used in high voltage overhead transmission lines are typically bare conductors of aluminum with a central core of steel to give strength and durability to the relatively weak aluminum conductor. These cables are termed Aluminum Conductor Steel Reinforced (ACSR) or Aluminum Conductor Steel Supported (ACSS) cables. These cables are produced in over seventy standard sizes and are generally code named after North American birds such as Linnet, Drake, Cardinal, and Joree. FIG. 1 is an illustration of a bare conductor cable 100. A multiplicity of aluminum conductor wires 101 are stranded around a central support core 102 which is also a stranded cable with between typically 7 and 19 steel wires.
Advanced composite technologies such as glass and carbon fiber, and metal matrix composites are possible replacements for steel in the supporting core. The rationale for replacing the steel with a composite core is to give better qualities to the transmission line system. In particular replacing the steel core with glass and carbon fiber composites is intended to give a lower temperature coefficient of expansion for the cable. A low temperature coefficient of expansion is important because when the overhead conductor carries a high current load the cable becomes hot and the cable expands. When the cable expands the sag between the support towers increases. When bare conductor cables are strung from the support towers they are designed to have a sag that is safely above the surrounding topography including trees and buildings under conditions of high current high temperature operation. The process of setting the tension and the sag of the conductors is called sagging. The lower the thermal expansion, and the smaller the change in the sag of the line under high load, the more economic it becomes to run at higher line voltages and currents as the safety margins are greater.
A sagging of a cable in a transmission line is illustrated in FIG. 2. The sagged cable 201 is supported between towers 202a and 202b above the terrain 203. The sag, D, 204, is set by changing the tension in the line. The sag, D, 204 is approximately related to the horizontal tension, H, 205, the span of the cable, S, 206, and the weight per unit distance of the cable, w, by the equation:D=H/w [cosh(wS/2H)−1]  (1)                which is approximated by the equation:D=wS2/8H  (2)        These equations are called the catenary equations.        
As the conductor heats up with increased current flow the cable expands and the tension in the cable decreases. As the horizontal tension, H, 205 decreases the sag D 204 increases. The increased sag means that the cable might contact trees or buildings under the cable, i.e., transmission line and the transmission line towers must be tall enough to prevent this happening.
The chosen cables must also have sufficiently high ultimate tensile strength to meet safety factors under conditions of high winds and winter icing in geographical areas that experience these conditions. Also the sag of the cable under these conditions must also not be large.
Glass fiber and Carbon fiber support cables are typically pultruded using a variety of resin binders. These cables can have a variety of shapes and compositions. These cables typically have a lower coefficient of thermal expansion than a steel cable. Steel has a coefficient of thermal expansion of approximately 10 ppm per degree Centigrade while E-CR glass has a coefficient of thermal expansion of 6 ppm and Carbon composite has a coefficient of thermal expansion less than 1 ppm.
Composite supporting cores using glass and carbon fibers have been proposed for a number of years and a commercial version of the technology is being marketed with carbon fibers. The quoted advantages of the composite core support are higher ultimate strength, lower thermal coefficient of expansion, and lower weight. However, composite core technology has a number of difficulties that have slowed the technology's acceptance into the mix of technology solutions available to the utility companies.
Among these difficulties is the low modulus of elasticity of the supporting cores. The low modulus of elasticity is particularly true for glass fiber based cores. Glass fibers have a lower modulus of elasticity than the steel used in ACSS. When glass fibers are combined with resin in a composite core, the resulting modulus of elasticity can be less than a third of that of steel.
Thus it would appear to those skilled in the art that apart from the higher ultimate strength and the lower thermal coefficient of expansion the low modulus of elasticity can seriously undermine the value of the composite technologies, particularly the value of glass fiber. Indeed the low modulus of elasticity for a glass fiber core suggests that to get to the higher ultimate strength of core the sag on a transmission line would be unacceptably large.